In most aircraft there are at two main gear, one on either side of the fuselage, and a nose gear, all retractable. It rarely occurs that the gear actuation system fails and the landing gear must be lowered manually, i.e., bypass the normal gear actuation system, which is hydraulic. In order to lower the landing gear manually, it is necessary to release the gear uplocks which hold the gear up and locked and to release the doors which cover the gear in the up and locked position. In addition, it is often necessary to release hydraulic fluid to allow the landing gear to move to the down position.
Past systems have included complex mechanical systems with cables entrained about pulleys or cable quadrants and driven by a single control lever.